Your salmon sandwiches in the interval might turn out to be a mistake. In the closing minutes of the first half of Harrison Birtwistle’s 2008 opera, the crazed half-man, half-bull of Greek myth rapes his victims and gores them to death in front of a baying mob. A squawking pack of harpies — think Goth vultures with the physiques of a champion lacrosse team — descend to rip out the hearts of the wounded and devour them whole. It’s understandable that no one quite knows whether to clap or to simply file out, stunned, into the foyer.